Television system



Jun4, 1940. v c. L; RICHARDS TELEVISICN SYSTEM Filed Dec. 28, 1957 2 Sheets-Sheet 1 INVEN TOR. C'ZAl/DE LANGDUN RICHARDS ATTORNEY.

June 4, 1940. c. 1.. RICHARDS TELEVISION SYSTEM Filed Dec. 28, 1937 2 Sheets-Sheet 2 INVENTOR 624005 [Al/60 0 RIC/MBA? ATTORN EY Patented June 4, 1940 UNITED STATES PATENT OFFICE TELEVISION sYs'rEM Claude Langdon Richards, Eindhoven, Netherlands, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application December 28, 1937, Serial No. 182,139

, In the Netherlands December 30, 1936 3 Claims. (01. ts- 7.5);

The invention relates to a television receiving installation and more particularly. to that part of the installation which serves to synchronize a television receiver with the transmitter to be received. e

For the purpose of synchronizing a transmitter and a receiver it has previously been suggested to transmit simultaneously with the picture current on the same carrier wave synchronizing impulses which are modulated in opposite sense on the carrier. wave and which are utilized at the receiver for controlling the scanning movement.

In order to avoid that at thelreceiver the devices which control the scanning movement are troubled by the picture currents received, it is customary to separate the picture currents from the synchronizing impulses prior to the supply of these impulses to one or more devices which control the scanning movement.

The invention has for its object to provide a television receiving system wherein a new circuit arrangement is employed for separating from one another synchronizing impulses and picture currents which are modulated-in oppositesenses on acarrier wave. 3

m The novel feature of the circuit arrangement according to the invention consists in. the prothe synchronizing impulses, is equal tothe amplification obtained when the non-modulated carrier wave is received.

The rectifiedoutput voltage of the additional high or intermediate frequency amplifier should preferably be supplied to an electrode of one or more tubes present in said amplifier, said tube or tubes beingadjusted in such manner that at the "minimum instantaneousvalue of the picture currentsthis tube or these tubes operate at a point of the characteristic curve which is located at the boundary between a substantially straight and a highly curved portion.

. The invention will be explained more in detail with reference to the accompanying drawings wherein:

. Figure 1 representsdiagrammatically one embodiment of a complete television receiver according to the invention.

Figure 2 represents one mode of realisation of that part of the receiver according to the invention which is denoted in Fig. 1 by I9.

Fig. 3 is an explanatory curve.

Figure 4 is an explanatory curve.

Figure 5 shows another embodiment of my invention.

Fig. 6 is an explanatory curve.

Fig. 1 represents a television receiver adapted to receive synchronizing impulses which are transmitted together with the picture currents and which bring about a decrease of the carrier wave amplitude, wherein a cathode ray tube is employed as a reproducing device. At the receiver the modulated high frequency oscillations received by an aerial are supplied to a first detector land transformed by means of a local oscillator 3 into modulated intermediate frequency oscillations. The modulated intermediate frequency oscillations are amplified in an intermediate frequency amplifier 5, detected in a second detector 1 and the picture currents and synchronizing impulses thus obtained are supplied through a low frequencyamplifier 9 to the control electrode II of a cathode ray tube It. The picture currents received control the intensity of the cathode ray beam generated in the tube I3 while the synchronizing impulses set up at the control electrode I I serve to block the cathode ray beamduring the duration of time between the scanning of two successive picture lines or pictures.

The cathode ray tube comprises two devices which control the scanning movement of the cathode ray beam. In these devices, which are denoted by l5 and Il respectively, is set up a sawtooth current or voltage. The device [5 controls the movement of the cathode ray beam in the direction of the picture lines (hereinafter referred to as horizontal scanning movement); the device Il controls the movement of the cathode ray beam in a direction perpendicular to the picture lines (referred to hereinafter as vertical scanning movement).

In order to ensure that the scanning movement of the cathode ray beam at the receiver is in synchronism with the scanning movement at the transmitter, the synchronizing impulses received are supplied to the devices I5 and Il via a device 19 by which the picture currents are separated from the synchronizing impulses. In the connection of the device I9 with the vertical scanning device ll is provided a filter II which only trans quency is equal to the'frequency of the horizontal scanning movement.

In the embodiment shown in Fig. 2 the device l9, which serves to separate picture currents and synchronizing impulses from one another, com: prises an intermediate -frequencyi amplifying tube 2 to the input circuit], of which are supplied modulated intermediate frequency oscillations which are taken from the output circuit of the first detector I (Fig. '1). The output circuit 6 of the tube 2 is inductively coupled with a circuit 8 which is connected in series with a diode Ill and an impedance constituted by the parallel connection of a resistance 12 and a condenser I4.

. The voltage across the impedance l2, I4 is supplied, if desired via a low-frequency amplifier which is not shown in the drawings, to the devices I5 and I1. The-voltage across the impedance [2, M is supplied furthermoreto the grid of the tube 2 owing to the fact that that point of the impedance 12, Which is located on the anode side of the diode I 0 is connected viaa-filter It by which only intermediate frequency oscillations are returnedto the'grid of the tube 2.. It is thus achieved that the grid bias of the intermediate frequency amplifying tube 2, which bias is determined by the voltage drop in the impedance l8 which consists of the parallel connection of a resistance and 'a condens'er, also depends on the instantaneous value of the'voltage across the impedance 12, M, in such manner that with an increasing carrier wave amplitude the grid bias becomes more negative and that with a decreasing amplitude the the intermediate frequency oscillations supplied to the grid of the tube 2, that is to say'the instantaneous value of the voltage V2 increases with the reception of the picture currents whichare denoted by 20 whereas the instantaneous value of the voltage V2 is only small withthe reception of the synchronizing impulses, which are denoted by22. The voltage across the impedance I2, M with the reception of a non-modulated carrier oscillation is determined by a dotted line 24. For the purpose of separating the picture currents from the synchronizing impulses, the bias Voltage V1, of the tube 2 is so chosen that with-a nonmodulated carrier oscillation the tube 2 operates at a point of the characteristic-curve which is, located at the boundary betweena highly curved portion a and-a substantially straight portion b of the iw-V characteristic curve With this adjustment the amplification of the tube 2 decreases, during-the reception of the picture currents with an increasing amplitude of the modulated oscillations received-whereas during "the reception of the synchronizingimpulses ,the'amplificationof the tube 2 is substantially equal to than, the carrier ave amplitud ever, the amplitude of the volta ge oneam nthat with the reception of the non-mod, .ated car'- rier Wave. In the output circuit of the intermediate frequency amplifier 2 there are consequently set up, intermediary frequency oscillations whose amplitude is substantially constant during the occurrence of the picture currents whereas the synchronizing impulses bring; about ;a strong decrease of the carrier wave amplitude; During the occurrence of the, picture currents therectiiied output voltage of the tube 2 is therefore substantially constant owing to which the synchronizing impulses alone are supplied to the devices l5 and I1,

A different embodiment is shown in Figure 5 wherein similar elements are denoted by the same references. The difference with the preceding system consists int-hat a second diode 26 is provided to which the intermediate frequency oscillations aresupplied via a coupling condenser 28.

The voltage across the output impedance of the diode 26, which impedance'consists of a-resistance 30 and acondenser 32, is supplied to thegrid of the tube 2-via' a filter l6 which preferably consists of a resistance 34 and a condenser 36 and1by which only intermediate frequency oscillations are retained. Thediode 26 has a negative bias (referred toas threshold value) which is equal to the non-modulated carrier 'wave amplitude. Across the output impedance 30, 32 of the diode '26 there will consequently be set up a'voltage characteristic curve-of the tube 2-whichmay'be utilized inthe system according to Fig. 5.---=T he bias-voltage of the tube 2: is determined" on the one hand by the voltage drop across the impedance l8, which is denoted by-V1, and on the other hand by the voltage -V2' across the impedance 3D, 32, which voltage depends on the instantaneous-value of the picture currents received. Duringthe occurrence of the picturecurrents the negativeeridbias of-the tube 2-increases with an increasing amplitudeof the oscillations to be amplified which are suppliedto-the grid whereas during the-occurrence (of the .synchronizing impulses. the negative bias =-has I the same value as with. the reception-of the-nonmodulated carrier oscillation; Againwith this in system it is consequently ensured that during the occurrence of the picture currents the amplitude i of the intermediate frequencyoscillations set up in the output circuit of the-amplifier 2 is substantially constant and that the synchronizing im- 2 pulses bring about a strongdecrease-of (the am- :plitude.

the device l9 (Fig. l) as a function of the interi mediate frequency input voltage .of said device,

the envelopin curve of the-intermediateirequenoy oscillations to be amplified being, repres t yi 111 henonrmod lated carrier wave amplitude 1 is determined by ,thei

dotted line, 36. It appearsffrom thisjcurvefthat the low. frequency output yo'ltage inerea'sesfwith an increasingamplitude or theintermediate frequency voltage tobe amplified aslofng "the am:-

plitude of the voltage to" be amplified sv smaller "10 Whemjhowfiedsurpasses'saidvalue, the amplification o'f ithe device lg decreasesin dependence, on'ithef finstantaneous 'value' of the modulation and the low frequency output voltage remains subs-tam tially constant; In the output circuit of the device I9 there will consequently be set up a substantially constant voltage which depends chiefly on the synchronizing impulses.

What I claim is: v

1. Television receiving apparatus for receiving video signals and synchronizing signals, the latter modulating the carrier wave in the opposite sense to that which the video signals modulate said carrier wave, comprising cathode ray means for reproducing the televised image from said video signals, means forldeflecting said cathode ray, means for energizing said deflecting means, and means for controlling the action of said defleeting energizing means comprising a thermionic valve operated on a curved portion of its grid voltage-plate current curve, means for developing a control voltage representative of the instantaneous modulation value of saidcarrier, means for impressing the received signals onto a control element of the first-mentioned valve,

and means for feeding a portion of the signal indicative of the instantaneous modulation value of the carrier to a control element of the firstmentioned valve, whereby for video signals the amplification of said valve decreases with an increasing amplitude of the modulated carrier, and

wherein said means for developing a control voltage representative of the instantaneous modulation value of said carrier comprises a time constant circuit including a condenser and a resistance, and a uni-directional conductor connected serially with said time constant circuit, and

means for impressing the output of said thermionic valve on to said uni-directional conductor.

3. Apparatus in accordance with claim 1 wherein said means for developing a control voltage representative of the instantaneous modulation value of said carrier comprises a time constant circuit including a condenser and a resistance, and a uni-directional conductor connected serially with said time constant circuit, and means for impressing the output of said thermionic valve on to said uni-directional conductor, and in which said uni-directional conductor comprises a diode.

CLAUDE LANG-DON RICHARDS. 

